Name: ____________________________________
Geologic Time – Two Column Notes
Use pages 229 – 232 in the Earth Science textbook to fill in the two-column notes below.
Big Idea
Details from the text
The Geologic
Column
 The ordered arrangement of rock layers is called a geologic column.
 A geologic column represents a timeline of Earth’s history.
 The oldest rocks are at the bottom of the column.
 Rock layers are determined by the type of rock and the kinds of
fossils the layers contain.
How do Scientists
use a geologic
column
 Scientists use geologic columns to estimate the ages of rock layers
that cannot be dated radiometrically.
 Radiometric dating is absolute dating, allowing scientists to
determine the ages of rock layers with more accuracy.
 To determine the age of a rock layer, scientists compare the rock
layer with a similar layer in a geologic column that contains the same
fossils. If the layers match, they likely formed at about the same
time.
 Geologists use markers such as changes in the Earth’s surface,
climate and types of organisms to divide the geologic time scale into
smaller units. A unit of geologic time is generally characterized by
fossils of a dominant life form.
 Ma stands for million years.
Divisions of
Geologic Time
Eons
Eras
Periods
 The largest unit of geologic time is an eon.
 Geologic time is divided into four eons: the Hadean, the Archean,
the Proterozoic and the Phanerozoic.
 The first three eons are part of a time interval known as
Precambrian. There are very few fossil from Precambrian rocks.
 Eons are divided into eras.
 The present geologic era is the Cenozoic Era which began about 65
million years ago.
 Eras are divided into units called periods.
 Each period is characterized by specific fossils and is usually named
for the location in which the fossils were first discovered.
Epochs
 Periods are divided into epochs.
 Epochs may be divided into ages.
(over)
Questions:
1. Compare the terms geologic time and geologic column.
A geologic column is a column of rock layers that is used in relative dating. Geologic time represents the sections that
Earth’s history is divided into.
2. How would our understanding of Earth’s past change if a scientist discovered a mammal fossil from the
Paleozoic Era?
It would mean that mammals evolved much sooner than scientists currently believe.
3. Use the following terms to create a concept map: geologic time, Precambrian time, Paleozoic Ear, Mesozoic Era,
Cenozoic Era, period and epoch.